The present invention relates to systems for and methods of angular measurement in tracking radar units capable of operation in a two-target situation, which unit comprises at least three primary radiators or feeds, and wherein means are provided to produce a sum signal, together with two difference signals and an additional cross-term signal from echo signals received by the primary radiators or feeds in a sum-difference network, when operating.
In target tracking and target homing with the help of radar devices, the multi-target problem is particularly important. This problem arises when a plurality of closely adjacent targets exists, since these must be separately distinguished by any efficient radar device. If one single pinpoint target lies within the radar units of resolution, whose accuracy is governed by the beam width and by the transmitted pulse duration, then conventional tracking radar systems do in fact permit highly accurate target resolution within this illuminated radar volume without further precautions and in a modern monopulse radar unit the angular resolution readily reaches values around one hundredth of the beam width. However, as is known, in such systems the angular measurement accuracy drops by one or more orders of magnitude as soon as there is a plurality of pinpoint targets, or one single pinpoint target which is no longer punctiform in appearance, lying within the radar scanned volume. The latter situation is referred to as a target "glint" and cannot be clearly detected, whereas the former situation will be specifically referred to in the following under the term "multi-target problem". Thus two or more targets can, in the absence of further measures, be incorrectly indicated as one single target by the radar device. When tracking multiple targets, the radar devices consequently continuously respond to a fictive target which may lie outside the volume to which the homing is directed, as determined by the various targets. If the target separation is not effected in adequate time, it is not possible for an effective firing control to take place.
Particular prominence in the multi-target problem is assumed by the situation in which two targets are apparently present, which may both be real flying objects in the radar volume under surveillance, or one single real flying object whose reflection simulate a second, virtual flying body via a smooth surface reflective to micro-waves.
This second situation is known as the mirror effect, and is a problem particularly to be considered when working over water surfaces. A characteristic of the mirror effect is that the real target and the mirror image always lie on a normal to a mirror surface, and thus in the case of a water surface, on a vertical line. Moreover, any changes in target and mirror image are closely linked to one another, which is manifest in a very small range difference and in a scarcely distinguishable Doppler shift. In contrast, in a normal two-target situation there is normally no relationship between the two targets.
German Patent Publication No. 1,900,854 discloses a method of recognising the simultaneous existence of two or more closely adjacent radar objects in the directional diagram of a radar antenna system. This method is provided for phase monopulse processes and requires at least five receiving antennae for a two-target situation and (2n+1) receiving antenna for any n-target situation, which leads to an uneconomically high expenditure in respect of receiving antennae.
Swiss Pat. No. 592,887 and the corresponding U.S. Pat. No. 4,084,160 discloses a method for correcting such mirror effect in target tracking radar units comprising at least three primary radiators or feeds, of which at least two are aligned in such manner that a plane passing through the longitudinal axes of their radiation characteristics is at least approximately at right angles to the reflective surface, a sum-difference network being employed to provide a sum signal and two difference signals, together with a cross-term signal formed in at least one measuring interval, these being produced from echo signals received by the primary radiators.
In accordance with this proposed known method, an angular or angle error signal broken down into two components E.sub.KQ, E.sub.KP is produced from these signals, and a signal processing stage provides a correction signal F.sub..DELTA. corresponding to the formula: EQU F.sub..DELTA. =-E.sub.EQ .multidot.E.sub.KP /E.sub.KQ +F.sub..DELTA.R
in order to correct any angular error signal known per se which has been adulterated by mirror effects. In this formula:
E.sub.EQ =the angular error signal component falsified by mirror effects in quadrature to the sum signal;
E.sub.KP =the angular error signal component in phase with the sum signal, derived from the cross-term function;
E.sub.KQ =the angular error signal component in quadrature to the sum signal derived from the cross-term function;
F.sub..DELTA.R =the residual correction.
Although a process of this kind is highly suitable for correcting those errors in target tracking radar devices that are caused by reflective surfaces, it does not take into account the general two-target situation.